Pets could be animals or minor monsters. I think hirelings should be humanoid. Minions should be magical creatures and constructed creatures, (et Golems).
I honestly see nothing wrong with a minor NPC humanoid hireling.
They cant start off strong at all.
They should be able to progress at say, 1/8 a normal characters speed.
They should not be able to participate in combat, I know this seems odd, but it will make sense next.
They should be able to perform odd tasks. A quartermaster would be like a mule. He could loot items for you from creatures you (or group) killed, and stow them with his many backpacks and satchels, (2.0 multiplier to any storage space items). A chanter could constantly chant for healing from his god in combat to bestow a small regeneration effect to a player or group, (non stack with other chanters). A professor could be the intelligent advisor to the fighting party, advising them to weaknesses of the monsters, (constant addition to damage, also shout out things like, \"use silver or fire against zombies\") You get the idea.
Also, they should constantly cost. Every IRL week, you have to pay a fee to keep their services.
You can train and upgrade them with instruction from wiser NPCs hidden in obscure places. You can give them items to modify their skill.
Etc etc. You could really do nice things with it if you really try.